Generally speaking, for a conventional scheme, a portable device such as a smart phone device cannot obtain voltage information actually from a battery cell included within a battery pack. The portable device is merely capable of obtaining a voltage level from an external port of the battery pack. Usually, there exists a significant voltage drop between the voltage level from the external port of the battery pack and a voltage level obtained actually from a battery cell. Thus, it is inaccurate to refer to the voltage level from the external port of the battery pack for charging or discharging the battery cell. The conventional scheme may cause a longer waiting time period for charging the battery cell to be full and a poor rate for battery power utilization. In addition, the conventional scheme fails to provide solutions for solving the problems of temperature change, the battery cell being removed, and etc.